Light controlled light modulator

ABSTRACT

A light controlled light modulator is disclosed wherein a low intensity, circularly polarized driving light beam can be used to modulate or switch a linearly polarized main light beam. The main beam can be as much as three orders of magnitude more powerful than the driving beam, yet the response time is measured in picoseconds. The modulator itself comprises a coherent rotator and a polarizer, and modulation is accomplished by rotating the plane of polarization of the main beam under the control of the driving beam. The coherent rotator is a completely passive device and is a light cell containing an alkali metallic vapor such as rubidium or potassium.

UIllwfl Mate;

Armstrong et al.

LIGHT CONTROLLED LIGHT MODULATOR Inventors: John A. Armstrong, SouthSalem; Daniel R. Grischowsky, Peekskill, both of N.Y.

International Business Machines Corporation, Armonk, N.Y.

Filed: Dec. 28, 1973 Appl. No.: 429,337

Assignee:

References Cited UNITED STATES PATENTS 6/1972 Duguay 350/157 X LINEARLYPOLARIZED ttzw Feb. 4, 1975 Primary Examiner.lohn K. Corbin Attorney,Agent, or Firm-Sughrue, Rothwell, Mion, Zinn &' Macpeak [57] ABSTRACT Alight controlled light modulator is disclosed wherein a low intensity,circularly polarized driving light beam can be used to modulate orswitch a linearly polarized main light beam. The main beam can be asmuch as three orders of magnitude more powerful than the driving beam,yet the response time is measured in picoseconds. The modulator itselfcomprises a coherent rotator and a polarizer, and modulation isaccomplished by rotating the plane of polarization of the main beamunder the control of the driving beam. The coherent rotator is acompletely passive device and is a light cell containing an alkalimetallic vapor such as rubidium or potassium.

9 Claims, 5 Drawing Figures M A|NBEAM I TTHRCULARLY POLARIZED DRlVlNGBEAM \POLARIZER 1 LIGHT CONTROLLED LIGHT MODULATOR BACKGROUND OF THEINVENTION 1. Field of the Invention The present invention generallyrelates to modulators for lasers and the like, and, more particularly,to an extremely fast, low loss, completely passive light modulator inwhich one light beam controls the passage of a second light beam.

2. Description of the Prior Art and Light modulators are known in whichthe modulation is effected by the use of the electro-optical effect.Typically, an optical element consisting of an electrooptical materialsuch as potassium dihydrogen phosphate (KDP) is used. In such materials,the index of refraction is changed by the application of an electricfield. The modulating electric field may be supplied by a microwaveenergy source, for example. Light passing through the light cellundergoes a change of velocity as a result of variations in therefractive index which produces a phase modulation in accordance withthe ra- I dio-frequency input signal.

It is also known that some optically active substances rotate thepolarization plane of polarized light passed through them. Othersubstances devoid of this property acquire it when subjected to a strongmagnetic field. This is known as the Faraday effect and is the principleof operation of some light modulators.

Common to light modulators based on either the electro-optical effect orthe Faraday effect is the requirement for modulation sources ofsubstantially high power. Furthermore, construction of these modulatorsis sometimes complicated owing to the need to couple electromagneticenergy into the electro-optical mate rial or place the optical substancein a strong magnetic field.

Many investigators have long been aware of the method of optical pumpingfor producing nonequilibrium population distributions of atoms in theirground and excited states. Zeeman pumping is one optical pumpingprocedure which is relevant to this disclosure and is described inProgress in Optics, edited by E. Wolf, Vol. V, pp. 5-8, published in1966 by North-Holland Publishing Company, Amsterdam, Holland. Thismethod produces unequal populations in the different magnetic sublevelsof the ground and excited states of atoms. This type of pumping ispossible because of the existence of quantum mechanical selection rulesconcerning the absorption of circularly polarized light which propagatesalong the magnetic field direction. Zeeman pumping can be done for atomswhich show the Zeeman effect which consists in the splitting up of thespectral lines of a substance'when subjected to the influence ofamoderately intense magnetic field.

Recently, investigations have been made of phenomena which can bedescribed by adiabatic following. These phenomena are reported, forexample, by D. Grischkowsky and J. A. Armstrong, in Self- Defocusing ofLight by Adibatic Following in Rubidium Vapor, Physical Review A, Vol.6, No. 4, October 1972, D. Grischkowsky, Adiabatic Following and SlowOptical Pulse propagation in Rubidium Vapor, Physical Review A, Vol. 7,No. 6, June 1973, and D. Grischkowsky, Eric Courtens and J. A.Armstrong, Observation of Self-Steepenlng of Optical Pulses withPossible Shock- Formation, Physical Review Letters, Vol. 31, No. 7, Aug.13, 1973. The term adiabatic following describes the situation in whichthe pseudomoment of the near-resonant transition follows the effectivefield of a laser pulse. Adiabatic following occurs when two conditionsare met. First, in the rotatingcoordinate frame, the direction of theeffective field must change slowly compared to the precession frequencyof the pseudomoment about the effective field; and second, the pulsewidth must be short compared to the relaxation times of the atomicsystem.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide an extremely fast, low loss, completely passivelight modulator which operates on the principle of the control by acircularly polarized driving light beam of the population densities ofatoms in the magnetic sublevels corresponding to the a" or 0* transitionof the ground and excited states of an atomic vapor in a light cell.

According to the teachings of the invention, a light cell is providedwhich contains a gas of one of the alkali metals such as rubidium andpotassium. This light cell is referred to as a coherent rotator and,under the control of the driving light beam, is effective to rotate theplane of polarization of a linearly polarized main light beam. Byproviding a polarizer at the output of the coherent rotator, theresulting structure is that of a highly efficient, simple and veryrugged light modulator capable of operating as a shutter which opens inless than I psec or as an amplitude modulator at frequencies as high as3 X 10 Hz. Moreover, the main beam can be as much as a thousand timesmore powerful than the driving beam. Two modes of operation are possiblede pending on whether the driving pulse is applied adiabatically ornon-adiabatically.

BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of the invention,as well as other objects, aspects, uses and advantages thereof, willclearly appear from the following description and from the accompanyingdrawings, in which:

FIGS. Ia and lb illustrate the light modulator and its operationaccording to the invention; and

FIGS. 2, 3 and 4 are energy level diagrams of atoms contained in thecoherent rotator of the modulator shown in FIGS. la and lb.

DESCRIPTION OF THE PREFERRED EMBODIMENT The modulator according to theinvention comprises the combination of a coherent rotator and apolarizer. As shown in FIG. la, without the driving beam the rotationangle 4:, of the polarization of the linearly polarized main beam isindependent of time. The polarizer is oriented at right angles to theplane of polarization of the main beam, and as a result, the main beamis completely blocked by the polarizer. The presence of the circularlypolarized driving beam in the coherent rotator changes be, and some ofthe main beam passes through the polarizer. When d is rr/2, the mainbeam is completely transmitted by the polarizer, as shown in FIG. lb.

The coherent rotator contains atoms (or ions, molecules, excitons, etc.)described by one of the three energy diagrams shown in FIGS. 2, 3 or 4.For the purposes of this description, each level is assumed to be doublydegenerate or exhibits two magnetic sublevels in each energy state. Theworking levels 1 and 2 are connected by and o circularly polarizedelectric dipole transitions, where level 2 is the atomic ground state.In addition, the level A in F IG. 3 is any level connected by 0* and 0"transitions to level 1, and the level B in FIG. 4 is any level connectedby 0* and a transitions to level 2.

Let the number density of atoms in the +95 or state'of level 2 bedesignated as N, or N, respectively. Similarly, let the number densityof atoms in the +r or state of level 1 be designated as N,* or N,',respectively. In the absence of the driving beam, all the atoms are inthe ground state 2, and N; N M2, where N is the atomic number density.

The frequency 11,, of the circularly polarized a" driving beam isresonant or nearly resonant with the frequency separation v, between thetwo working levels 1 and 2. The frequency v,,, of the linearly polarizedmain beam is relatively far from resonance from any one of the threetransition frequencies 1 v, or u corresponding to FIGS. 2, 3, or 4,respectively. In the following example, we assume v,, is closest to v,and consider the energy level diagram of FIG. 2.

The rotation angle of the polarization of the main beam caused bypassage through the coherent rotator is given by where n (n') is theindex of refraction for the 0* (0') component of the main beam; 1 is thelength of the coherent rotator; k is the wavelength of the main beam. Inthe limit that the Zeeman splitting approaches zero, the equation for (bcan be written as where P is the absolute value of the matrix element ofthe electric dipole moment operator for the at transitions between theworking levels 1 and 2. By factoring out common terms, equation (2)becomes When a a driving beam is supplied to the coherent rotator, theeffect of the a driving beam is to put the atoms into a superposition ofthe a" ground and excited states, which in turn changes (Nf-Nf). In thenotation of coherent optics (Nf-Nf) -(-N/2)cos0, where 0 is the anglebetween the pseudomoment and the direction of propagation (positive 2axis) of the driving beam. (NJ-N N/2is unchanged by the driving beam.Thus, with the driving beam present, Eq. 2 becomes If the driving beamsatisfies the conditions of adiabatic following, an explicit expressionfor cos 0 can be written e050 Iv -v l where E,,, is the peak electricfield of the linearly polarized main beam.

As an example, consider rubidium vapor as the coherent rotator material.Let the working levels correspond to the P line (7,948A) of RbnLet(v,,-vd)/c 1 cm; thus, the switching speed I, is approximately 30 psec.Assume 0.= at the peak of the controlling pulse. As P 6.16 X 10' e.s.u.for this transition, 0 120 requires a peak intensity for the drivingbeam of only 400 Kw/cm If |v,,,v,, /c 100 cm", then setting E,,,/2 u -vl 0.25 E l lv v l we see that the main beam can be more than onethousand times as intense as the driving beam. if we let the length ofthe coherent rotator be 100 cm (a reasonable value), we calculate thatthe atomic number density N l.3 X l0/cm is required to have (b 1r/2 atthe peak of the controlling pulse. This number density corresponds to aRb cell temperature of only C, which is easily ob tained. For amodulator and a driving beam with these parameters, the driver cancompletely open and close the modulator in about 30 psec, can keep themodulator open, and can put any slowly varying (compared to 30 psec)amplitude modulation on the powerful main beam.

It is interesting to compare this value of (15 1r/2 with the rotationangle p obtained from the Faraday effect with the same cell in amagnetic field H. In this case, the rotation angle 1b; of thepolarization of the main beam due to the Faraday effect caused bypassage through the rotator is given by which can be expanded as (b (1rlP N/A h) l/(v,,,v,-8) l/(u,,,v,,+8)}

where 28 is the Zeeman splitting between the 0* and 0 lines. Because 8|v,,,v,|, is approximately equal to Consider 26 1 cm; this correspondsto a magnetic field H 8 kG, which is a relatively large field. Thus, forthis example di /d); 50. This shows that the coherent rotator gives anextremely large rotation compared to other devices.

cise, we assume E was applied suddenly at t and then held constant, thenand Eq. 4 for becomes Thus, du is oscillating, and the modulatorwilltransmit a train of pulses with the-repetition rate P E /fi. it isimportant to notice that the repetition rate is proportional to themagnitude ofthe driving field; the modulator converts amplitude changesof the driving field into changes in thepulse repetition rate.

This time for a'numerical example, consider potassium vapor which has avery small hyperfine splitting. Now 11, :1 increase the intensity of thedriving beam to 4 Mw/cm. This choice. gives I7 P E /hc 5.5 cm". Let|v,,.v,|/c i0 cm, then for the maximum value of durr/2, N==0.65 X 10 /cmwith l 100 cm.

If we had chosen to have a faster switching time, the power required forthe driving beam would have been increased. it should be realized thatthe frequencies 1 and u... can be completely different if the modulatoruses the energy level scheme of FIG. 3 or FIG. 4. For example, a weakvisible light beam could control the passage of a 10 micron C0, laserbeam of enormous power. Furthermore, while a ocircularly polarizeddriving beam was assumed for the specific examples given, a 0 circularlypolarized driving beam could be used.

It will be apparent that the embodiments disclosed are only exemplaryand that various modifications can be made in construction andarrangement within the 6 scope of the invention as defined in theappended claims.

What is claimed is:

l. A light controlled light modulator comprising:

a. a coherent rotator containing a material exhibiting Zeeman splittingof the optical spectra of radiations emitted by atomic transitionswherein the population densities of atoms in the Zeeman magneticsublevels of the ground and excited states are controlled by acircularly polarized driving light beam, and

b. a polarizer oriented to variably transmit a main beam of linearlypolarized light as a function of the rotation of the plane ofpolarization of the main beam by passage through said coherent rotator.

2. A light controlled light modulator as recited in claim 1 wherein thecoherent rotator material in said coherent rotator is selected from theclass of alkali metals..

3. A light controlled light modulator as recited in claim 2 wherein thecoherent rotator material is rubidrum.

4. A light controlled light modulator as recited in claim 2 wherein thecoherent rotator is potassium.

S.- A light controlled light modulator as recited in claim 1 wherein thefrequency 11,, of the driving beam is resonant or nearly resonant withthe frequency separation v, between two working levels of the atomicstates of the atoms in said coherent rotator material and the frequencyv, of the main beam is far from the frequency separation v, betweensaid'two working levels.

6. A light controlled light modulator as recited in claim 5 wherein 11,,v, and a pulse of the driving beam is applied adiabatically.

7. A light controlled light modulator as recited in claim 6 wherein thecoherent rotator material is rubidium vapor.

8.'A light controlled light modulator as recited in claim 5 wherein v11,, and a pulse of the driving beam is applied non-adiabatically.

sium vapor.

a at a: a:

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 864,020

DATED February 4, 1975 |NVENTOR(S) I John A. ARMSTRONG et 3.1

it is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below;

IN THE SPECIFICATION:

Column 1, line 10 after "Art" delete "and" Column 3, line 59 delete"N/Zis" and insert N/Z is Column 4, line 21 delete P" and insert P zline 21 delete 7, 948A)" and insert 7, 948A Column 5, line 30 delete he"and insert h Signed and sealed this 29th day of April 1975.

(SEAL) Arrest: C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attest ing of ficer and Trademarks

1. A light controlled light modulator comprising: a. a coherent rotatorcontaining a material exhibiting Zeeman splitting of the optical spectraof radiations emitted by atomic transitions wherein the populationdensities of atoms in the Zeeman magnetic sublevels of the ground andexcited states are controlled by a circularly polarized driving lightbeam, and b. a polarizer oriented to variably transmit a main beam oflinearly polarized light as a function of the rotation of the plane ofpolarization of the main beam by passage through said coherent rotator.2. A light controlled light modulator as recited in claim 1 wherein thecoherent rotator material in said coherent rotator is selected from theclass of alkali metals.
 3. A light controlled light modulator as recitedin claim 2 wherein the coherent rotator material is rubidium.
 4. A lightcontrolled light modulator as recited in claim 2 wherein the coherentrotator is potassium.
 5. A light controlled light modulator as recitedin claim 1 wherein the frequency Nu d of the driving beam is resonant ornearly resonant with the frequency separation Nu o between two workinglevels of the atomic states of the atoms in said coherent rotatormaterial and the frequency Nu m of the main beam is far from thefrequency separation Nu o between said two working levels.
 6. A lightcontrolled light modulator as recited in claim 5 wherein Nu d < Nu o anda pulse of the driving beam is applied adiabatically.
 7. A lightcontrolled light modulator as recited in claim 6 wherein the coherentrotator material is rubidium vapor.
 8. A light controlled lightmodulator as recited in claim 5 wherein Nu d Nu o and a pulse of thedriving beam is applied non-adiabatically.
 9. A light controlled lightmodulator as recited in claim 8 wherein the coherent rotator material ispotassium vapor.